This invention relates to distance sensors, and more particularly to sensors which receive bounced radiated signals from an object, whose distance from the sensor, velocity or acceleration is to be measured.
Many different ways to measure distances are known. Items such as rulers, yard sticks and tape measures can be used where short distances, generally under one hundred feet, are involved and where it is possible to physically get to the two locations between which a measurement is to be made. However, there are some instances where use of one of these devices is impractical, for example over long distances, where access is not possible, or where measurements are to be made such a great number of times that human participation is undesirable.
To overcome this problem, distance sensors which radiated an object with a signal were created, such as radar and ultrasonic sensors. The both radar and the ultrasonic sensors operated on a similar principal. A base unit created and transmitted a signal toward an object whose distance was to be measured. The signal was reflected by the object back to the base unit which then determined how much time elapsed between transmission of the signal and return of the signal. The signal propagation rate was known or could be calculated, thus leaving a simple calculation to determine the distance between the base unit and the object.
However, the sound or radar signal may bounce off of objects other than the one to be measured thus providing an erroneous return signal. In addition the shape and structure of the object may cause a complete loss of signal so that no measurement can be made.
To this end, light based sensors have been developed. One well known version of a light based sensor are laser speed detectors used by the police to determine the speed of vehicles. These laser speed detectors work in substantially the same way as described above in that the time of travel from the base unit to the object and back to the base unit is used to determine the distance of the object, and two such readings are used to determine velocity.
While this represented an improvement over the previously described detectors, problems still remained. For example, it is difficult over small distances to determine travel time of the laser signal to the object and back due to the speed of the laser signal. This difficulty causes such detectors to be complex and expensive.